Note to Teachers:
For several classroom activities related to separations, click here.

7A (Reading and Thought Experiments)

Pure Substances, Mixtures, and Separations

The Basics of Separation

One way scientists talk about matter or substancethat is, the stuff in the worldis in terms of pure substances and mixtures. Pure substances are substances that contain only one kind of molecule. Water with nothing else in it is a pure substance. This is because it contains nothing but water molecules. On the other hand, suppose we put a teaspoon of sugar in a glass of water and let it dissolve. Now we have a glass full of mostly water molecules with some sugar molecules here and there between the water molecules. So our sugar-water isn’t a pure substance. It consists of more than one kind of molecule; thus it is a mixture.

Separation in the Real World

Mixtures are all around us, and most of the things you can touch or feel are mixtures, not pure substances. A lot of times this isn’t a problem. Orange juice is a mixture of water, sugar, vitamin C, citric acid, and more, and it’s perfectly fine as a mixture. But sometimes we need to separate the different substances in a mixture. For example, milk is a mixture of water, sugar, fat, protein, vitamins, calcium, and more. Calcium and protein are two things your body needs, but fat, while good for growing children, can be unhealthy for adults. To make a healthier milk for adults, we separate most of the fat from the milk, creating low-fat or “skim” milk.

Crude oil (petroleum) is a mixture of all kinds of substances. Some of the substances are used to make gasoline and other fuels. Others are used to make plastics. Still others are used to make medicines. It takes a lot of work to separate all these substances in crude oil from each other. This is done in giant factories called refineries.

Soybean oil is also a mixture, and the challenge Percy Julian faced at the Glidden Company was to separate and isolate useful substances from it. He separated various types of steroids from soybean oil (see 8. Steroids from Soybeans, which were then converted into helpful drugs. He also separated proteins from soybeans that could be made into plastic. Julian separated still other protein compounds that he used to make a new kind of fire extinguisher.

Whether we’re talking about milk, crude oil, or soybeans, separation of mixtures is very important in our world.

Thought Experiments

In the thought experiments that follow, you'll be asked to think about ways to separate mixtures. In the classroom, your teacher may actually carry out one or all of these activities so you can see whether your ideas work. Your teacher may also conduct more complicated lab activities, in which you can conduct various types of separations. (Teachers:See more activities.)

Click on the links to try these three thought experiments. They will get you thinking about some of the basic concepts behind the process of separating substances from each other.